In order to characterize the quantum entanglement of the emitted photon pair, we use the so-called concurrence as a measure of the entanglement [1]}. The concurrence can be calculated from the density matrix as
\(\mathcal {C}(\rho _{\mathrm {f}})=\mathrm {max}(0,\sqrt{\zeta _1}-\sqrt{\zeta _2}-\sqrt{\zeta _3}-\sqrt{\zeta _4})\;,\) 
